1. Field of the Invention
The present invention relates generally to the field of animal breeding and the production of animal food products. More particularly, it concerns methods for ranking and selecting animals for feed efficiency.
2. Description of Related Art
Little genetic improvement for meat quality or the efficiency of production has occurred in beef cattle populations in the last 100 years, despite development of Selection Index theory over 60 years ago (Hazel, 1943). This is due at least in part to the little information available on which to make selection decisions to improve these traits. It is time consuming, difficult, and costly to obtain carcass information in commercial packing plants and to retain the identity of individual animals. Thus, little information is available upon which to make breeding decisions to improve the net efficiency of growth. Considerable efforts have been expended to develop live animal ultrasound techniques to provide indirect measures of carcass traits. Due to the importance of these traits and their cost and difficulty of measurement, there is a great need for development of measures for selection of beneficial traits in beef cattle such as diagnostic methods based on biochemical and genetic markers. Such techniques could greatly increase the productivity of breeding programs and eliminate the need for costly or ineffective phenotypic selections.
Expected Progeny Differences (EPD), a genetic evaluation tool, have gained increasing use in cattle breeding. Many purebred beef and dairy cattle organizations now conduct yearly evaluations that calculate EPD for a number of cattle weight, growth, and production traits with economic importance, including birth weight, weaning weight, ribeye area, and others. However, an EPD for feed efficiency or a trait that is strongly correlated with feed efficiency has not yet been developed, in part because a recognized standard for “efficiency” has been lacking. Efficiency may be defined in a number of ways. Ratios of inputs and outputs, such as gain to feed (G:F) or feed to gain (F:G), also termed the “Feed Conversion Ratio” (FCR), have been used. However, as noted below, these ratios can confound growth rate, body size, and appetite with metabolic efficiency.
One promising approach for developing a feed efficiency EPD involves Residual Feed Intake (RFI), sometimes called net feed intake. RFI is defined as the difference between an animal's actual feed intake and its expected feed requirements for maintenance and growth. Thus, it is the variation in feed intake between animals that remains after requirements for maintenance and growth have been removed. Expected feed intake is calculated based on the statistical modelY=β0+β1X1+β2X2+ε,
wherein Y is expected feed intake; β0 is a regression intercept; β1 is the partial regression of daily feed intake on average daily gain (ADG); X1 is Average Daily Gain; β2 is the partial regression of daily intake on body weight; X2 is body weight; and ε is the random error. The body weight of the animal is typically expressed as the midweight during test (sometimes transformed to a “metabolic midweight” by raising the midweight to about the power of 0.75, e.g. kg0.75; Crews 2005). The RFI for an animal is calculated as actual feed intake minus expected intake (Y). The mean RFI for a tested population is zero. Efficient animals, with an RFI below zero, have daily feed intakes below what would have been predicted given their levels of production or body weight.
Importantly for breeding purposes, RFI has been found to exhibit moderate genetic heritability. However, given the phenotypic way in which it is calculated, the underlying biochemical and genetic factors that result in a given RFI have been unclear. RFI has typically been calculated by an expensive and time consuming phenotypic process, wherein cattle are subjected to a feeding regimen, and their individual feed intake and growth are closely followed, typically over a more than 70 day period, for instance, in conjunction with use of a feed management system like the GrowSafe® Feed Intake System (U.S. Pat. No. 6,868,804), or other cattle management system (e.g. U.S. Pat. No. 6,805,075), in order to obtain data on their feed intake and growth. Significantly, RFI may be used as a selection tool that does not confound metabolic efficiency with growth rate.
Johnson et al. (2003) and Herd et al. (2003) discussed dietary energy use research in beef cattle production in general, including the use of calculated RFI as an efficiency measurement. Basarab et al. (2003) reported differences in average daily feed intake (ADFI) and G:F (gain to feed ratio) when steers grouped according to their calculated RFI were compared. Basarab also reported increased fat deposition in steers selected to have high RFI. However, mitochondrial function was not examined. Nkrumah et al. (2004) reported on the relationship between RFI and other measures of energetic efficiency and growth in cattle. However, no underlying mechanism to account for variations in RFI between animals was demonstrated.
Moore et al (2005) found that insulin-like growth factor (IGF) was correlated to residual feed intake (genetic correlation of 0.35). The less efficient cattle had higher IGF levels, as would be expected since these cattle consume more feed without increased levels of gain. Use of this approach to select or predict cattle for feed efficiency can however be influenced by the feeding management scheme of the calf, and is not as highly correlated to RFI as mitochondrial respiration rate. Owens et al. (1996; WO96/35127) also describe selection of livestock (e.g. pigs) based on IGF levels.
Bottje and coworkers (Bottje et al. 2002; Iqbal et al., 2004; Ojano-Dirain et al., 2004; WO03/032234) describe aspects of mitochondrial function, including level of reactive oxygen species production, that may be used to select for “feed efficiency” (FE) in broiler chicks. They reported that activity of mitochondrial complexes I and II was positively correlated with FE in broiler chicks. That is, high FE birds had higher respiratory chain complex activities than lower FE birds. However, Bottje and coworkers did not use RFI as a measurement of feed efficiency. Rather, their definition of FE, e.g. as a ratio consisting of the weight gain of an animal divided by the weight of feed consumed (G:F), or its inverse (F:G), confounds several underlying variables, including growth rate, body size, and appetite, with the metabolic efficiency of feed use, per se.
Bottje and coworkers (e.g. Bottje et al., 2002; Iqbal et al., 2004; Ojano-Dirain et al., 2004) reported increased weight gain for chickens displaying high feed efficiency, with no difference in feed intake between high and low feed efficient birds, and have also reported that isolated mitochondria of low feed efficient chickens generated greater amounts of hydrogen peroxide than did mitochondria of high feed efficiency birds. Lutz and Stahly (2003) have also described evidence of a link between inefficient mitochondrial respiration and decreased G:F in rats.
Sandelin et al. (2004) reported that activities of respiratory chain Complex I and II were higher in low FE steers than in high FE steers. They also use G:F as their measure of FE. The result in cattle apparently contradicts the work of Bottje et al., above in chickens. Thus, the relationship of mitochondrial function as measured by mitochondrial protein activities, rate of electron flux through the electron transport chain, and correlation to FE (however defined) in animals is unclear.